A fuel cell is a power generation system that converts chemical energy, obtained from a reaction between oxygen and hydrogen from a hydrocarbon-based material, such as methanol, ethanol, or natural gas, to electrical energy.
A fuel cell can be classified into a phosphoric acid type, a molten carbonate type, a solid oxide type, a polymer electrolyte type, or an alkaline type of fuel cell depending upon the kind of electrolyte used. Although each fuel cell basically operates in accordance with the same basic principles, the kind of fuel, the operating temperature, the catalyst, and the electrolyte may be selected based on the type of cells.
Recently, polymer electrolyte membrane fuel cells (PEMFC) have been developed with superior power characteristics compared to conventional fuel cells, lower operating temperatures, and faster starting and response characteristics. Such technology has advantages in that it can be applied to a wide array of fields such as transportable electrical sources for automobiles, distributed power sources for houses and public buildings, and small electrical sources for electronic devices.
The polymer electrolyte fuel cell is essentially composed of a stack, a reformer, a fuel tank, and a fuel pump. The stack forms a body, and the fuel pump provides fuel stored in the fuel tank to the reformer. The reformer reforms the fuel to generate hydrogen gas and supplies the hydrogen gas to the stack. The hydrogen gas is electrochemically reacted with oxygen in the stack to generate electrical energy.
The fuel cell may also be a direct methanol fuel cell (DMFC) type in which liquid methanol fuel is directly introduced to the stack. The direct methanol fuel cell can omit the reformer which is essential for a polymer electrolyte fuel cell.
According to the above-mentioned fuel cell system, the stack substantially generating the electricity has a structure in which several or several tens of unit cells, each consisting of a membrane electrode assembly (MEA) and a separator (referred to as a “bipolar plate”) are laminated together. The membrane electrode assembly is composed of an anode (referred to as “fuel electrode” or “oxidation electrode”) and a cathode (referred to as “air electrode” or “reduction electrode”) separated by a polymer electrolyte membrane.
A conventional membrane-electrode assembly has a structure where a high-density polymer electrolyte membrane directly contacts the catalyst layers. The catalyst layers are formed by being coated on the polymer electrolyte membrane directly, or by being coated on a microporous layer formed on one side of a gas diffusion layer (GDL).
However, when the catalyst layer of the membrane-electrode assembly is formed very thin through an evaporation method, and laminated to a high-density polymer electrolyte membrane, the catalyst layer may be surrounded by the high-density polymer electrolyte membrane causing a problem from increased diffusion paths during the gas transfer. Consequently, there may be problems in that the gas transfer rate is low and that the water generated at the cathode clogs the pores in the gas diffusion layer.